The so-called Vampire catfishes of the trichomycterid genus Vandellia attack the major gill arteries and don't suck out the blood of their prey, according to the results of a new paper.
Two Brazilian fish biologists, Ivan Sazima and Jansen Zuanon, studied two parasitic candiru called Vandellia cirrhosa and V. sanguinea, which are known to attack the gills of large catfishes. However, it wasn't known how or where the fish attached themselves when inside the gill cavity of their host.
Their evidence suggests that these two fish go straight for the major gill, dorsal and ventral arteries. Once they've punctured the artery of their host it's thought they they simply feast on the blood as it's pumped out under the pressure of the host's circulatory system.
The scientists conclude that the notion of candirus being blood suckers is actually misleading: "We suggest that candirus do not need any special sucking or pumping mechanism become rapidly engorged themselves with blood but simply use their needle-like teeth to make an incision in an artery."
Related fishes have also been recorded swimming into the urinary tract of humans. (Don't click this link if you are squeamish!)
For more details see: Zuanon, J and I Sazima (2004) - Vampire catfishes seek the aorta not the jugular: candirus of the genus Vandellia (Trichomycteridae) feed on major gill arteries of host fishes. Aqua, 8: 31-36